This invention relates to methods and apparatus for limiting the amount of electrical current supplied to a load, and more particularly for limiting the amount of current to a subsea electrical coupler.
There are many applications where it is desirable to limit the electrical current supplied to a load. In many instances, an ordinary circuit breaker may be sufficient to limit the electrical current to a predetermined value. The fact that the circuit breaker may require manual operation to return the circuit to an operative condition may be desirable, since it may be important to realize that the current to the load has exceeded a certain value.
In other applications, it may be beneficial to automatically return the circuit to an operative condition after the current has been limited. This may be accomplished by providing that the circuit breaker return supplied power to the load after a certain interval of time. For instance, if the current to a load exceeds a preselected value and the circuit is broken, the circuit breaker may automatically return the power to the load two seconds after the circuit has been broken. At that time, if the current requirement of the load is within the normal operating range, the circuit is successfully reset. If, on the other hand, an abnormally high current is still being drawn by the load, the circuit breaker will again interrupt the power to the load.
A major disadvantage of using a circuit breaker in series with the load as a current limiting device, as described above, is that after the circuit has been broken, there is no convenient way to predetermine if the load conditions are such that current should be restored. Also, if the load requirement remains excessively high, the response time for the circuit breaker may be such that damage may nevertheless occur because the load was briefly subjected to excessive current.
Methods and apparatus were therefore desired which would safely and accurately limit the current supplied to a load while providing for automatically re-establishing the current supply upon return of the load conditions to normal values. The problem, of course, is complicated when the electrical load and the current limiting devices are located in a remote and hostile environment. For instance, in applications where the current limiting device is located in a subsea environment, a relatively simple yet highly reliable arrangement should be employed in view of the high cost of repairing and/or replacing such a device. For this reason, subsea applications often dictate that electrical devices employing moving parts and electrical contacts should not be utilized. Further, electrical apparatus utilized in subsea applications are often contained within hermetically sealed enclosures for excluding the seawater and protecting the apparatus against moisture and corrosion, and this presents additional problems if the electrical apparatus is to be repaired or replaced.
In a subsea petroleum production installation, three phase electrical power is typically distributed to numerous satellite wells from a production control manifold. The electrical supply cable connecting the satellite wells to the production manifold may be miles long, and may include one or more inductive power couplers. Simply stated, one of these inductive power couplers is a transformer, and each half of the coupler is sealed to protect the windings from the subsea environment. Inductive couplers allow the electrical supply cable to be effectively mated to or disengaged from an electrical load at this satellite well, without exposing the current conductors to the hostile subsea environment.
A coupler may become accidentally or purposefully disengaged at a time when power is being supplied to the satellite well. Accidental disengagement of the power coupler may result from the limited dexterity of men and equipment working in a subsea environment. Purposeful disengagement most often occurs when either the electrical supply cable is disconnected from one satellite well to be used on a different well, or when a portion of the electrical equipment is disconnected and removed to the surface for repair, service or replacement. When the power coupler is disengaged, the current drawn by that half of the coupler being supplied with power will increase to undesirably high levels, and if the current is not limited that half of the coupler being supplied with power may fail because of excessive heating.
If the current is to be limited to protect the power coupler, the current limiting device should have the capability of restricting the current to a magnitude equal to or less than the maximum non-destructive current capacity of the inductive coupler. Additionally, it is important that as long as the current being supplied to the power coupler is within the maximum amperage of the coupler, the current limiting device should not seriously restrict the power flow to the satellite well or introduce significant power dissipation. In other words, the current limiting device should act to limit the current to a preselected value, but should not introduce appreciable impedance as long as the current is equal to or below a preselected value.
If the power coupler, once becoming unmated and thereby causing the current limiting device to restrict the current to the coupler, later becomes re-mated so that normal load current would be drawn through the power coupler, the current limiting device should be capable of automatically sensing this condition so that the current limiting function will be effectively removed from the supply line. Thus, the current limiting device should have automatic reset capability responsive to the actual load conditions.
As stated earlier, power commonly is distributed from a subsea production manifold through several miles of supply cable to a satellite well. Typically, one power coupler is located at the control module of the satellite well, and the control module essentially is the load for that supply cable. As explained below, it is beneficial, that the current limiting device be located at the production manifold. First, the production manifold is a convenient location to mount the current limiting device. There often is not a location along the electrical supply cable or the satellite well where the current limiting device may be so easily mounted and/or serviced if repair is ever required. Second, having the current limiting device at the production manifold allows the electrical supply cable to be moved to different satellite wells without disturbing or relocating the current limiting device. Most importantly, it is more economical to service, repair or replace the current limiting device if it is conveniently located at the production manifold rather than at the satellite well.
In addition to the difficulties previously discussed relating to the application of current limiting devices in subsea environments, further problems are encountered in providing this current limiting function for a subsea petroleum production installation as described above. First, the high cost of subsea petroleum production installations require that production down time be kept to a minimum, and that, as much as possible, service of equipment be performed on a regularly scheduled basis. Second, it is beneficial that each electrical supply cable be connected to a single current limiting device although the supply cable may include multiple power couplers. A single current limiting device, as compared to multiple current limiting devices for each power coupler, also increases the reliability of the overall system since there are fewer components within the system. However, since the electrical power cable may be miles long, the power couplers may be located long distances from the current limiting device, and it is not practical to provide control circuits extending from the power couplers to the current limiting device. Third, the production manifold must be capable of supplying full electrical power to various other satellite wells at a time when it is necessary to restrict the power to a specific satellite well because a power coupler along its respective electrical supply cable is uncoupled.
The problems and disadvantages of the prior art are overcome with the present invention. Novel methods and apparatus are hereinafter provided for limiting the electrical current to a load, and, more particularly, for limiting the electrical current to a subsea petroleum installation.